Ultrasound imaging creates images of the inside of the human body by broadcasting ultrasonic energy into the body and analyzing the reflected ultrasound energy. Differences in reflected energy are made to appear as differences in gray scale or color on the output images. As with other medical imaging procedures, contrast enhancing fluids (other referred to as contrast media) can be injected into the body to increase the difference in the reflected energy and thereby increase the contrast displayed in the image (that is, the image contrast) viewed by the operator.
For ultrasound imaging, the most common contrast media contain many small bubbles (sometimes referred to as microbubbles) suspended therein. The difference in density of bubbles when compared to water, and thus their difference in sound transmissivity, makes small gas bubbles excellent means for scattering ultrasound energy. Small solid particles can also serve to scatter ultrasonic energy. Such particles are typically on the order of 1 to 10 microns (that is, 10.sup.-6 to 10.sup.-5 meters) in diameter. These small particles can pass safely through the vascular bed and, in some cases, traverse the pulmonary circulation. Contrast agents are also used for non-vascular applications such as assessment of tubal patency in gynecological applications.
Contrast media suitable for use in ultrasound are supplied in a number of forms. Some of these contrast media are powders to which liquid is added just before use. The powder particles cause gas bubbles to coalesce around them. The powder must be mixed with a liquid, and the mixture must be agitated with just the right amount of vigor to obtain the optimum creation of bubbles. Another type of contrast medium is supplied in a liquid form that requires hypobaric or pressure activation. A third type of contrast medium is a liquid that is agitated vigorously. There are no solid particles to act as nuclei, but the liquid is a mixture of several liquid components that make relatively stable small bubbles. A fourth type of contrast medium uses "hard" spheres filled with a gas. These contrast media are typically supplied as a powder that is mixed with a liquid. The goal is to suspend the spheres in the liquid without breaking them. Even though such spheres have a shell that is hard compared to a liquid, they are very small and relatively fragile. It is also possible for the solid particles themselves to act to scatter ultrasonic energy, but the acoustical properties of the solid spheres are not as different from water as those of a gas. However, solid particles have the advantage that they are much more robust and long lasting.
Suspended or dispersed entities such as microbubbles (liquid or gas), microspheres and solid particles suitable to enhance ultrasonic imaging contrast are referred to herein as contrast enhancement agents/particles. With these agents there are several problems, including: (1) Variations in the preparation process (mixing, agitation, pressure activation) can lead to variations in microsphere or particle concentration that affect the resulting imaging procedure. (2) Some agents deteriorate with time after preparation, causing the concentration of the microbubbles or particles to decrease; (3) microbubble-agents can also be adversely affected by pressure before or during administration, causing microbubble destruction by increasing gas diffusion rates or damage to the encapsulation shell from delivery or pressure effects. This may also affect the microbubble concentration.
Contrast enhancement agents also enhance other modes of ultrasonic imaging. For example, when the microbubbles, microspheres or particles are carried along in the blood stream, the reflected energy is Doppler shifted. This Doppler shift allows an estimation of the speed of blood flow. Bubbles can also be excited so that they radiate ultrasonic energy at an harmonic of the incident ultrasonic energy. This harmonic imaging with the use of contrast medium can be used to increase the effectiveness of the contrast agent.
After mixing/preparation as described above, the contrast medium is drawn into a syringe or other container for injection into the patient. Typically, the fluid is injected into the vein in the arm of the patient. The blood dilutes and carries the contrast medium throughout the body, including to the area of the body being imaged (that is, the region-of-interest or ROI). As mentioned above, the contrast medium can also be injected into other body cavities or tissues as necessary for diagnostic or therapeutic activities.
It is becoming more common for a microprocessor-controlled, powered injector to be used for injecting the contrast medium. The use of such powered injectors has the benefit of maintaining a consistent flow over a long time, thereby providing a consistent amount of contrast medium in the blood stream. If there are too few contrast enhancement particles per unit volume in the flow, however, there will insufficient image enhancement and the diagnosis cannot be made. If too many contrast enhancement particles are present, too much energy is reflected, resulting in blooming or saturation of the ultrasound receiver.
Thus, although a power injector can inject contrast medium at a constant flow rate, there must generally be a constant number of contrast enhancement agents per volume of fluid injected to provide a constant image contrast. Because a gas is significantly less dense than water and other liquids, however, gas bubbles will rise in a liquid. The rate of rise is related to the diameter of the gas bubble. This density difference provides a useful tool to quickly separate large bubbles created during the initial mixing. However, the small bubbles desired for image enhancement will also rise slowly. Solid particles, on the other hand, will tend to settle or sink because most solids are more dense than water. Many minutes can elapse between the initial mixing of the contrast medium and the injection into the patient, or the injection itself may be several minutes in duration. If the concentration of particles changes, the image contrast will be degraded as mentioned above.
There are also many other reason why the number of contrast enhancement agents per volume of a certain contrast medium (and thereby the image contrast) can vary during an injection procedure. For example, the initial mixing may not have resulted in a homogeneous dispersion or suspension. Likewise, bubbles or microspheres of certain contrast media can be destroyed under conditions experienced in mixing/preparation, storage or delivery of the contrast media.
It is, therefore, very desirable to develop systems and methods to control the concentration of contrast enhancing agents delivered to a patient in an ultrasound imaging procedure.